1. Field of the Invention
This invention relates to method for cancelling an impulsive noise in a system of transmitting a band-limited baseband signal through a channel with a wider band than that of the baseband signal.
2. Description of the Prior Art
In the transmission using a channel, it is of prime importance that transmitted information is faithfully regenerated in the receiving station. In practice, however, transmitted signals are disturbed by transmission distortion and noise in the channel; therefore the transmitted information is not always regenerated completely in the receiving station. To avoid this, there have been proposed a variety of transmission systems in which signals are not easily affected by such disturbance under the condition of a limited transmitting power. For example, in a microwave circuit, an SSB-FM system has been widely employed, and recently a digital communication system such as a PCM system has also come into use.
However, these conventional systems have a defect such that when a ratio C/N (the carrier power vs. noise power ratio) is smaller than a certain value, the signal quality is rapidly degraded by an impulsive noise. That is, when a ratio C/N is reduced to increase the noise amplitude, the signal is phase-modulated by the noise. If the noise amplitude exceeds the signal amplitude, a resultant vector phase varies within a phase range of 0 to 2.pi. because their phases change independently of each other. In this case, since it is an angular frequency that the phase is differentiated by time, the differentiated value of the resultant vector phase varies in the vicinity of the signal angular frequency. In a case where the phase of the received signal has been inverted by the noise, it changes from -.pi. to .pi. or vice versa. At this time, a jump of the phase occurs, so that the angular frequency becomes infinite to produce a large spike in the output from an FM detector which is employed for detecting a frequency deviation. For the SSB-FM system, a high sensitivity demodulation system which equivalently reduces the transmission band to increase a ratio C/N, such, for example, as a frequency negative feedback system (FMFB), a phase synchronization negative feedback system (PLL) or the like, offers an effective solution to the abovesaid problem. Further, for the digital communication system, there are various error correcting systems which provide redundancy in transmitted information.
However, these conventional methods introduce complexity in the circuit construction, and in the case of the latter, the amount of information being transmitted is inevitably reduced.